Mirror Modifier Issues - Asymmetry
By default the mirror modifier will choose its mirroring axes depending on the object's origin, not the world origin. 

The origin of each object is marked by a yellow dot when it is selected. As you can see, your tooth's yellow dot is a little bit to the left of it, so logically the mirror modifier creates a copy with a symmetry relative to its that.

If you want to create a mirror with a symmetry to the world origin, you have to select in the "Mirror object" field of the mirror modifier either:


An object which you know has their origin in the world origin 0, 0, 0
Create a new empty object, and keep it at the origin point
Make sure your 3D cursor is at the origin with SHIFT + C and while your object is selected, right click > Set Origin > To 3D Cursor